The present invention relates to a method and apparatus for well logging and particularly to a compensated measuring while drilling (MWD) well logging method and apparatus in which at least one receiving antenna is longitudinally positioned above at least one pair of transmitting antennas and at least one other receiving antenna is longitudinally positioned below the at least one pair of transmitting antennas.
A measurement of resistivity (or conductivity) is often performed as part of an investigation of formations surrounding a borehole. In particular, measuring the resistivity may be valuable in determining whether a particular formation surrounding the borehole contains hydrocarbons since it is known for example that a formation containing hydrocarbons typically has a higher resistivity than a formation containing salt water.
One class of devices for investigating formations surrounding a borehole determines formation resistivity by transmitting and receiving electromagnetic wave signals through the formation. In particular, one of such devices involves transmitting an electromagnetic wave from a single transmitting antenna through the formation and receiving the electromagnetic wave using receiving antennas located at respective distances from the transmitting antenna.
However, phase shift and amplitude ratio errors may be introduced by very slight physical changes in the receiver antennas induced by changes in pressure and temperature. Other errors may be introduced if signals are received by different receiving antennas and/or processed by different receiver electronics. For example, two separate receivers which are intended to perform identically may induce errors in measurements due to differences therebetween. U.S. Pat. No. 5,402,068 issued Mar. 28, 1995 discusses upper and lower transmitting antennas being energized successively via separate and dedicated driver circuits proximate each transmitter. Measurements from the transmitting antennas can be combined to eliminate cancellation system error components. Amplitude effecting error components and phase effecting error components may be eliminated.
Another of such devices is capable of providing compensated resistivity measurements by arranging a pair receiving antennas between upper and lower transmitting antennas. However, this type of device often requires two transmitters (i.e., two transmitter electronics units), one connected to the upper transmitting antenna and the other connected to the lower transmitting antenna. Moreover, this type of device often further requires wires that respectively connect an energizing source with each of the transmitters. Since the wires often pass a nearby receiving antenna, noise may be detected by the receiving antenna as a result of the crosstalk from the wire(s).
There thus remains a need in the art for a well logging device and technique which remedy the above drawbacks. For example, it would be beneficial to provide a well logging device which minimizes errors caused by any possible crosstalk or any other source, improves accuracy of measurements, provides improved quality of logs by increasing frequency, and reduces overall power requirements.
U.S. Pat. No. 4,899,122 issued Feb. 6, 1990, U.S. Pat. No. 4,949,045 issued Aug. 14, 1990, and U.S. Pat. No. 4,968,940 issued Nov. 6, 1990 illustrate well logging devices which investigate formations surrounding a borehole and in particular well logging devices which include a pair of receiving antennas arranged between upper and lower transmitting antennas.
In an exemplary embodiment of the present invention, an apparatus and method for determining resistivity of a formation surrounding a borehole comprises a housing having a longitudinal axis; first and second transmitting antennas affixed to the housing for transmitting electromagnetic waves to the formation; a first receiving antenna for detecting the electromagnetic waves, the first receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing above the first and second transmitting antennas; a second receiving antenna for detecting the electromagnetic waves, the second receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing below the first and second transmitting antennas; and a processor operatively coupled to the first and second receiving antennas for determining the resistivity of the formation based on the electromagnetic waves detected by the first and second receiving antennas.
A drill bit may be arranged proximately to one end of the housing. The processor may determine the resistivity of the formation by determining at least one of a phase shift and an amplitude ratio between the electromagnetic waves detected by the first receiving antenna and determining at least one of a phase shift and an amplitude ratio between the electromagnetic waves detected by the second receiving antenna.
The first and second transmitting antennas may be alternately connected to a transmitter which energizes the transmitting antenna to which it is connected with a signal having one of a plurality of available frequencies. Alternatively, the first and second transmitting antennas may be alternately connected to a transmitter which energizes the one of the first and second transmitting antennas to which it is connected with a signal that simultaneously includes a plurality of frequency components. As another alternative, the first and second transmitting antennas may be respectively connected to first and second transmitters so that electromagnetic waves are simultaneously transmitted by the first and second transmitting antennas with different frequencies.
In another exemplary embodiment of the present invention, an apparatus and method for determining resistivity of a formation surrounding a borehole comprises: a housing having a longitudinal axis; first and second transmitting antennas affixed to the housing for respectively transmitting first and second electromagnetic waves through the formation; a first receiving antenna for detecting the first and second electromagnetic waves transmitted through the formation, the first receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing above the first and second transmitting antennas; a second receiving antenna for detecting the first and second electromagnetic waves transmitted through the formation, the second receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing below the first and second transmitting antennas; a third receiving antenna for detecting the first and second electromagnetic waves transmitted through the formation, the third receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing above the first and second transmitting antennas; a fourth receiving antenna for detecting the first and second electromagnetic waves transmitted through the formation, the fourth receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing below the first and second transmitting antennas; and a processor operatively coupled to the first, second, third and fourth receiving antennas for determining the resistivity of the formation based on electromagnetic waves detected by at least two of the first, second, third and fourth receiving antennas.
In another exemplary embodiment of the invention, an apparatus and method for determining resistivity of a formation surrounding a borehole comprises: a housing having a longitudinal axis; a first transmitting antenna affixed to the housing for transmitting a first electromagnetic wave through the formation; a second transmitting antenna affixed to the housing for transmitting a second electromagnetic wave through the formation; a third transmitting antenna affixed to the housing for transmitting a third electromagnetic wave through the formation; a fourth transmitting antenna affixed to the housing for transmitting a fourth electromagnetic wave through the formation, each of the first, second, third and fourth transmitting antennas having a different longitudinal position along the longitudinal axis of the housing; a first receiving antenna for detecting the first, second, third and fourth electromagnetic waves, the first receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing above the first, second, third and fourth transmitting antennas; a second receiving antenna for detecting first, second, third and fourth electromagnetic waves, the second receiving antenna being affixed to the housing at a position along the longitudinal axis of the housing below the first, second, third and fourth transmitting antennas; and a processor operatively coupled to the first and second receiving antennas for determining the resistivity of the formation based on at least two of the electromagnetic waves detected by the first and second receiving antennas.